I find it a bit odd that you apparently booked a complicate route via Göschenen when the direct Milan - Brig train is operating on April 1st.
So I guess you ran in to the same issue that so many people from abroad run in to. They book to early. The RE80 trains (which are basically just commuter trains you do not need to book in advance) get loaded in the timetable the moment SBB loads its timetable for the new year. The EC trains however get loaded whenever Trenitalia feels like it.
So is it the case possible that you tried to book when the RE80 was already in the system, but the EC not, and that the system them found you a route, and sold you a ticket for that route.
Had you waited a bit you would have been able to book Milan - Brig - Zermatt.